A Cache Valley-like virus (Bunyamwera-group), W-993, at low mouse brain passages, was transmitted by Anopheles quadrimaculatus, A. punctipennis, and Aedes vexans. Anopheles quadrimaculatus showed the greatest transmission efficiency, averaging 47% and 37%, respectively, when they ingested a dosage of 10 to the 5th power point 5 or 10 to the 5th power point 6 ICLD50/0.03 ml. Transmission efficiency was lower with the 7th and subsequent passages. The anopheles appeared capable of being biological vectors of Cache Valley virus in nature. In comparative laboratory tests, Aedes triseriatus, a known biological vector of LaCrosse virus (California-group), was a more efficient transmitter of the virus to suckling mice than were Aedes vexans, A. canadensis, A. aegypti, A. stimulans, A. trivittatus, Anopheles punctipennis or Culiseta inornata. Aedes hendersoni, after ingesting infectious bloodmeals containing 10 to the 4th power point zero to 10 to the 6th power point zero SMLD50/0.03 ml of blood, failed to transmit a lethal dose of LaCrosse virus to suckling mice. Efforts to colonize Aedes hendersoni (using induced copulation) have so far been successful through the F4 generation. Control of A. triseriatus by closure of the treehole breeding sites in an area narrowly separated from adjacent woodlands was not successful. A need for wider "barrier" zones was indicated.